An axial conversion gear device that converts a first direction of input torque into a second direction is known.
FIG. 4 illustrates a gear transfer device described in Japanese Unexamined Patent Publication No. 2010-101366 as an example of an axial conversion gear device. The gear transfer device includes an axial conversion unit 200 and a speed reduction unit 300. The axial conversion unit 200 includes a first bevel gear 210, a second bevel gear 220 engaged with the first bevel gear 210, and an input gear 230 that has a same axis as the second bevel gear 220 and that has an outer diameter smaller than the second bevel gear 220. On the other hand, the speed reduction unit 300 includes a center gear having a first outer gear 310 and a second outer gear 320 having a diameter greater than the first outer gear 310, a crankshaft having an outer gear 330 engaged with the first outer gear 310 and an eccentric portion 340, and an external tooth gear 350 that oscillates and rotates as the eccentric portion 340 oscillates and rotates. The second outer gear 320 of the center gear is engaged with the input gear 230 of the axial conversion unit 200, and in an axial direction of the input gear 230, the second bevel gear 220 is located farther from the first outer gear 310 than the input gear 230. In the gear transfer device, input torque of a motor is transferred to the second bevel gear 220 through the first bevel gear 210 so as to transfer the input torque to the axial direction of the input gear 230. The torque is transferred from the input gear 230 to the external tooth gear 350 through the center gear and the crankshaft. As a result, the external tooth gear 350 oscillates and rotates.
FIG. 5 illustrates a gear transfer device described in Japanese Unexamined Patent Publication No. 2010-159851. In this gear transfer device, an input gear 230 is located closer to a bottom surface B1 side than a second bevel gear 220 in an axial direction of the input gear 230. The input gear 230 and an outer gear 330 of a crankshaft are disposed nearly on a same plane.
In a gear transfer device described in Japanese Unexamined Patent Publication No. 2010-101366, since the second bevel gear 220 and the input gear 230 have the same axis, the gear transfer device can be prevented from radially becoming large. However, in this gear transfer device, the second bevel gear 220 is located farther from the first outer gear 310 than the input gear 230 in the axial direction of the input gear 230. Thus, the first bevel gear 210 engaged with the second bevel gear 220 and the motor that outputs torque to the first bevel gear 210 are located far from the first outer gear 310 in the axial direction of the input gear 230. A bottom surface B1 of the gear transfer device is designed for example based on a height of a side surface of the motor. Thus, if the motor is located far from the first outer gear 310 in the axial direction of the input gear 230, the bottom surface B1 is accordingly located far from the first outer gear 310. As a result, a dead space is formed between the second bevel gear 220 and the bottom surface B1 of the gear transfer device in the axial direction of the input gear 230. Consequently, the gear transfer device is likely to axially become large.
On the other hand, in the gear transfer device described in Japanese Unexamined Patent Publication No, 2010-159851, since the input gear 230 is located axially closer to the bottom surface B1 side than the second bevel gear 220, the gear transfer device can be prevented from axially becoming large. However, in this gear transfer device, since the input gear 230 and the outer gear 330 are located nearly on a same plane, the axes of the second bevel gear 220 and the input gear 230 have to be located apart from the axis of the crankshaft having the outer gear 330 to some extent so as to prevent the input gear 230 and the outer gear 330 from interfering with each other. Thus, it is difficult to radially downsize the gear transfer device.